The present invention relates to the selective operation of downhole tools as used in the oil and gas industry and in particular, though not exclusively, to a re-settable circulation tool operated by a drop ball mechanism.
While many downhole tools operate continuously through a well bore e.g. scrapers and brushes as disclosed in U.S. Pat. No. 6,227,291, it is more desirable to provide a tool which performs a function only when it has reached a preferred location within a well bore. An example of such a tool would be a circulation tool as disclosed in WO 02/061236. The tool provides a cleaning action on the walls of the casing or lining of the well bore. The cleaning action may be required after the casing has been brushed or scraped and thus the tool is designed to be selectively actuated in the well bore. Such tools provide the advantage of allowing an operator to mount a number of tools on a single work string and operate them individually on a single trip in to the well bore. This saves significant time in making the well operational.
Tools which are selectively actuable in a well bore commonly operate by having an element which can be moved relative to the tool when in the well bore. In the circulation tool of WO 02/061236, the element is a sleeve located in the cylindrical body of the tool. When run in the well, the sleeve is held in a first position by one or more shear screws. To actuate the tool, a drop ball is released from the surface of the well through the work string. On reaching the sleeve, the ball blocks the flow of fluid through the tool and consequently pressure builds up until the shear screws shear and the sleeve is forced downwards. The movement of the sleeve is then stopped when a lower ledge of the sleeve contacts a shoulder on the internal surface of the tool body.
Such tools have a number of disadvantages. The tools are generally limited to one actuable movement. If two sleeves are incorporated to overcome this, the shear screws of the second sleeve can operate prematurely under the shock created to shear the shear screws of the first sleeve. Additionally, the reduced bore diameter of the lower part also effects the flow rate achievable through the tool.
One tool which has been developed to operate repeatedly is that disclosed in U.S. Pat. No. 4,889,199. This tool comprises a tubular body having a radial port into which is located a sleeve having a matching radial port. The sleeve is slidably mounted and its action controlled from a deformable drop ball biasing the sleeve against a spring. Initially the spring biases the sleeve to a closed position in which the ports are misaligned. The drop ball causes the sleeve to move to a position where the ports align due to a build up of pressure behind the ball, and fluid is discharged radially through the ports. A small steel ball is then dropped into the tool which seals the radial ports and the consequential pressure build up extrudes the deformable ball through the ball seat. The steel ball will drop with the deformable ball and both are retained in a ball catcher at the base of the tool. When the balls drop together the spring biases the sleeve back to the closed position and the tool can be operated repeatedly.
A disadvantage of this tool is that it requires both a deformable ball and a smaller metal ball to operate. Care must then be taken to ensure the balls are dropped in the correct order. The smaller metal ball must lodge in the second, radial, outlet in order to stop flow and thus the tool is restricted to having a single radial port. This limits the amount of cleaning which can be performed.
It is an object of the present invention to provide a downhole tool which obviates or mitigates at least some of the disadvantages of the prior art.
It is a further object of at least one embodiment of the present invention to provide a circulation tool which is re-settable and operated by similar drop balls.
It is a further object of at least one embodiment of the present invention to provide an actuation mechanism to move a sleeve within a downhole tool.